1. Field of the Invention
The present invention relates to a roll stamper used for molding a substrate for an optical recording medium, which is used for forming convexity and concavity for preformat signals or guide grooves for tracking signals, on a substrate used for a high-density information recording medium such as an optical disk or an optical card. It also relates to a process for preparing the roll stamper, and a process for preparing an optical recording medium making use of it.
2. Related Background Art
Substrates for optical recording mediums have been hitherto prepared by methods such as injection molding and compression molding. These methods, however, can not achieve sufficient flatness or smoothness of a substrate and also tend to cause warpage or inclusion of bubbles, thus bringing about the problem that the detection of information with light may be greatly inhibited. To avoid such a problem, great difficulties are involved in conditioning temperature and pressure at the time of manufacture, assuring accuracy of a mold, or preventing generation of bubbles, and also an apparatus with a large scale is required, resulting in a great increase in cost.
Moreover, substrates are formed sheet-by-sheet in compression molding, and hence complicated post-processing is required, bringing about a problem of poor productivity.
On the other hand, use of a flat sheet made of plastics enables easy manufacture of a smooth and uniform sheet having no bubbles, and hence a method has been proposed in which a stamper is brought into close contact with a plastic flat sheet under application of pressure to transfer grooves to the sheet. However, a very high pressure must be used in order to apply pressure over the whole disc surface of a flat sheet.
As a means for solving this problem, a method has been proposed in which a plastic sheet extruded from an extruder is passed between a roll stamper and a pressure roll, grooves corresponding to the stamper are thereby formed on the plastic sheet under a low pressure, and then center holes and peripheries of groove patterns thus formed are cut away, whereby substrates for optical recording mediums can be continuously obtained.
As roll stampers used in such a method of continuously preparing substrates for optical recording mediums, it has been proposed to use a stamper comprising a roll substrate having preformat patterns directly formed thereon or a stamper comprising a thin stamper stuck on a roll substrate with an adhesive or the like, as disclosed in U.S. Ser. No. 186,145, filed Apr. 26, 1988, by Tetsuo Kuwabara, et al. Of these, the stamper comprising a thin stamper fixed to the roll substrate can be readily prepared, compared with the stamper having preformat patterns directly formed thereon, and has the advantage that, when the stamper is broken, the broken stamper may be replaced with a new one and the original roll stamper can be used again.
Use of this method, however, has involved the problem that, in contrast to a conventional flat sheet stamper, a strong force is always applied to the stamper provided on the roll substrate, in the transport direction of a resin sheet, so that the stamper tends to be peeled from the roll substrate or the stamper is stretched in the transport direction of the resin sheet.
As a conventional means for fixing a stamper to a roll substrate, it is known that, as described in Japanese Patent Application Laid-Open Sho 56-86721, the stamper is mechanically fixed with screws, or adhered with an adhesive. In the instance where the stamper is fixed by a mechanical means, a gap tends to form between the roll substrate and the stamper because of the stretch of the stamper. Thus, there has been a problem in flatness. In addition, when a preformat is transferred to a plastic sheet, the roll stamper is heated with a heating medium inserted to the center of a roll substrate. Then, there is another problem that the heat conduction to the stamper may be non-uniform because of the gap between the roll substrate and the stamper and hence the preformat can not be readily transferred.
In the instance where the stamper is adhered with an adhesive, the heating of a roll stamper may cause deterioration of the adhesive, and hence there is also a problem in firmly fixing the stamper to the roll substrate for a long period of time. Moreover, no adhesives have been available which can absorb the expansion of the stamper without deteriorating due to heat.